The platinum metals (ruthenium, rhodium, palladium, osmium, iridium, and platinum) have outstanding catalytic properties. The chemical industry, for example, uses a significant amount of either platinum or a platinum-rhodium alloy catalyst in the form of gauze to catalyze the partial oxidation of ammonia to yield nitric oxide, which is the raw material for fertilizers, explosives, and nitric acid. In recent years, a number of platinum-group metals have become important as catalysts in synthetic organic chemistry. Ruthenium dioxide is used as coatings on dimensionally stable titanium anodes used in the production of chlorine and sodium hydroxide. Platinum supported catalysts are used in the refining of crude oil, reforming, and other processes used in the production of high-octane gasoline and aromatic compounds for the petrochemical industry. Since 1979, the automotive industry has emerged as the principal consumer of platinum-group metals. Palladium, platinum, and rhodium have been used as oxidation catalyst in catalytic converters to treat automobile exhaust emissions. A wide range of platinum-group metal alloy compositions are used in low-voltage and low-energy contacts, thick- and thin-film circuits, thermocouples and furnace components, and electrodes.
Characterization of the binding and/or reaction of adsorbates at the surfaces of platinum group metals is critical for understanding and thus improving their catalytic reactions. However, current methods of studying these reactions are less than satisfactory. Electron energy loss spectroscopy (EELS), for example, is highly satisfactory in detecting molecules bound to model surfaces, but suffers from the need for expensive equipment and ultra-high vacuum environments. Hence, it cannot be used to replicate actual behavior of catalytic reactions that occur at or above atmospheric pressure or in the presence of a liquid. Fourier-transform infrared spectroscopy (FTIR), often coupled with the use of attenuated total reflectance (ATR) techniques and Raman spectroscopy, have also been used to characterize adsorption processes, but the technique is not surface selective and signal-to-noise ratios often suffer from solvent effects. Moreover, with the growing trend toward greener aqueous-based reactions, the technique is especially limited by the strong IR absorption of the O-H stretching modes of water. Additionally, current techniques often require integration times much longer than actual reaction times.
Thus, there is a critical unmet need for sensitive, surface-selective, and easy-to-implement methods to provide detailed molecular-level information on heterogeneous catalytic reactions while they occur under realistic reaction conditions.
Discovered in the late 1970's, surface-enhanced Raman spectroscopy (SERS) is a strong candidate to fulfill this need. Raman spectroscopy detects molecules with chemical bonds that exhibit changes in polarizability, and SERS provides orders-of-magnitude improvement in detection limit through the use of nanostructured metal substrates. The primary mechanism responsible for the surface enhancement is the ability of the metal to support directly excitable surface plasmons at the excitation laser wavelength and across the Stokes frequency range. This plasmon response provides an intense electromagnetic field at the metal surface at both the excitation and Stokes wavelengths.
The shape and size of the metal nanoparticles strongly affects the strength of the enhancement because these factors influence the ratio of absorption and scattering events. There is an ideal size for these particles—not just any small particles will have the same impact on the Raman intensity—as well as an ideal surface thickness for each experiment. Particles which are too large allow the excitation of multipoles, which are nonradiative. As only the dipole transition leads to Raman scattering, the higher-order transitions will cause a decrease in the overall efficiency of the enhancement. Particles which are too small, however, lose their electrical conductance and cannot enhance the field. Furthermore, when the particle size approaches a few atoms, the definition of a plasmon does not hold, as there must be a large collection of electrons to oscillate together.
With recent developments in understanding structure effects on plasmonic behavior in metal nanostructure synthesis, SERS substrates with higher and more highly reproducible enhancements can be designed. Engineered substrates based on nanometer-sized metallic shapes, like rods, rings, gaps, bowties, and shells offer SERS enhancements of up to 109. In particular, Au nanoshell (NS) SERS substrates have been successfully used to determine conformation of surface-bound biomolecules and to transduce the spectral signature of surface-bound thiols in a high-resolution alloptical pH nanosensing device. The large active area for SERS on nanoshell surfaces, along with the large and highly reproducible enhancements designed into these structures, provide a SERS-active substrate that is quite promising for monitoring catalytic processes. However, existing Au nanoshells (e.g., US2008204742 by Halas; US2008096289 and US2008096289 by Zhou; and U.S. Pat. No. 7,371,457 et seq by Oldenburg) have only demonstrated enhancements up to 107, so there is still room for improvement.
The use of SERS as a probe for adsorption and reaction of chemicals at catalytic surfaces has been attempted previously, with many groups attempting to overcome the inherent SERS limitation to coinage metals (gold and silver) and extend SERS to typical catalytic materials, such as palladium (Pd), rhodium (Rh), platinum (Pt) and iridium (Ir). Previous studies focused primarily on the study of electrochemical reactions, where the substrates typically were roughened Au electrodes coated with 1-2 nm of catalytic metal or covered with platinum-group metal nanoparticles. More recently, structured substrates consisting of solely of the catalytic material have been introduced. Overall, the techniques have enjoyed only moderate success, offering typical enhancements of 104-106.
What is needed in the art are ever more sensitive detection methods, devices and compositions for use in biological, spectroscopic and other applications. More sensitive methods can be advantageously employed in the monitoring of chemical processes, for example in-line monitoring of water remediation processes.